This section provides background information related to the present disclosure which is not necessarily prior art.
There are placed requirements on devices of the process measuring technology, which in several respects go beyond those of other measuring devices. These include the use over months and years in a rough process environment as uninterruptedly as possible, moreover simple and standardized exchange of information with the target systems (controllers, process control systems, etc.) as well as simple commissioning and maintenance (see V. Gundelach, L. Litz “Moderne Prozessmesstechnik”, 1999, p. 10). The process environment in which such a flowmeter operates is characterized in particular by its use at non-constant medium temperatures. It is therefore necessary to separate from the measured temperature difference between the sensor element and the medium that portion which results from the changed medium temperature and thus not by a changed heat dissipation due to a varying flow rate.
In contrast, measuring devices requiring the presence of a constant ambient temperature, such as a hot wire probe, are not applicable under the conditions mentioned. By way of example, reference is made to U.S. Pat. No. 6,453,739 B1. For these measuring devices a changed flow rate of the air stream as the sole cause for the detected heating or cooling of the heating element or hot wire can be assumed only at a constant ambient temperature.
In process measuring technology thermal flowmeters generally use a differential temperature measurement. A first measuring element generates a local temperature increase and measures the actual measuring temperature, wherein the measuring temperature results from the heating power of the measuring element, the temperature of the flowing medium and the flow-dependent heat transport capacity of the flowing medium. Furthermore, often a second measuring element measures a reference temperature. According to their function, the first measuring element is often referred to as a heating element and the second measuring element as a temperature element.
Typical applications are systems with two spatially separated measuring points for detecting the medium and the heating element temperature. Here, moreover, a distinction must be made between systems with constant heating power, in which the measured variable is the overtemperature, and systems with constant or regulated overtemperature, in which the measured variable is the heating power or a quantity derived therefrom. In the following, the disclosure is based on overtemperature-controlled calorimetric flowmeters.
Overtemperature-controlled calorimetric flow monitors are available as systems with two measuring elements—wherein one measuring element simultaneously serves as a heating element and a temperature sensor—and systems with two measuring elements and a separate heating element. The control in systems with only two measuring elements, as is the case, for example, in the abovementioned patent DE 10 2004 055101, is implemented analogous in that the target value for the overtemperature to be regulated depends on the equipment of a measuring bridge or the corresponding resistance values. The supply voltage of the bridge is then controlled so that the heating element assumes a value by self-heating such that the bridge voltage is equal to 0 volts. The disadvantage here is among others the temperature coefficient of the measuring bridge to be considered, since the target value for the overtemperature depends on the medium temperature.
Systems with two measuring elements and a separate heating element can be controlled digitally, so that disadvantages of the analog control can be avoided, however, the higher circuit complexity results in an increase of the production costs. Furthermore, the measuring speed in this system is not as fast as in a system with two measuring elements, since the heating field between the heating element and the temperature sensor requires a certain time for its propagation.